Drying is an important process in wood product manufacturing for a number of reasons including prevention of wood degradation (e.g., mold), reduction of weight for transport, production of a stiffer product, and reduction of fluctuations in dimensional changes. Most wood products are dried in some type of dry kiln. Modern dry kilns provide a controlled temperature and humidity environment and are equipped with fans and vent systems to force air circulation and ventilation.
Dry kilns may be categorized as one of two types: batch dry kilns or continuous dry kilns. In a batch dry kiln, stacked loads of wood products are loaded into the kiln to be dried in a stationary batch process in a single chamber. In a continuous dry kiln, stacked loads of wood products enter the green end of the kiln and are moved forward through multiple drying chambers until exiting the dry end of the kiln. Continuous kilns typically consist of three chambers: one main chamber and two conditioning chambers.
Heat to a kiln may be provided either by indirect means or by direct firing. When a kiln is heated directly, combustion (or flue) gases and/or hot air from a source exterior to the kiln are directed into the kiln. When a kiln is heated indirectly, a source exterior to the kiln typically produces a heated fluid which is circulated in equipment such as a heat exchanger. Both batch kilns and continuous batch kilns feature vent control systems, which let in dry air and discharge hot humid air.
Kiln drying is a high energy consuming process, which can account for approximately 70 to 90 percent of a facility's energy needs; therefore, heat recovery may be an attractive method for reducing a facility's drying energy costs. However, the value of energy saved is largely dependent on the differential temperatures between the exhaust temperature of the combustion unit and the kiln exhaust temperature, the efficiency of the recovery system, and its capital and maintenance costs. Thus, there is a need to develop new systems and methods for drying wood products in kilns.